1. Field of the invention:
This invention relates to the removal of static charge, and also dust and lint simultaneously from surfaces. In many industries the elimination of the static charge which accumulates on electrically non-conductive surfaces is of utmost importance. This static charge binds dust and lint particles to these surfaces and is counter-productive to the process of the particular industry. The buildup of static charge in capacitive bodies can also precipitate detrimental effects in many commercial areasm. An example of the former is the photo-processing industry, where positive prints are made from photographic negatives. It is necessary to keep these negatives free from dust and lint so that the dust and lint are not imaged on the prints making them unacceptable. An example of the latter is the electronic industry where static charge build-up can affect and damage sensitive solid state electronic components.
2. Description of prior art:
Over the years several methods have been devised to eliminate static charges by making the surrounding air electrically conductive thus allowing the static charges to be conducted away from the affected area. These include such techniques as vaporizers and atomizers to humidify the air, radio-active materials to ionize the air, and also high voltage emitters to ionize the air. Other methods which make direct contact with the surface have been used such as anti-static brushes and wipers and liquids which are applied directly to the surface.
Static charge on a non-conductive plastic surface develops usually as the result of contact with another plastic surface or item. Such pastic surfaces or items in contact have an atomic attractive force that holds them together. This force is electric in nature and is of the variety that holds materials together. Separation of the items results in a rending of some of the negatively charged electrons of the atoms from one of the surfaces by the strong attractive force of the other and the adherence of those electrons to that surface. Thus, the surface that has lost electrons is left with an electric charge to again attract negatively charged electrons, and has thereby acquired a positive charge. And, the surface which has gained a surplus of electrons has thereby acquired a negative charge.
This is a classic example of how static charges develop. However, static charges are known to develop in many ways and on surfaces and bodies that do not fit the above example. Static charges are transferable through conductive means such as in the Van de Graff generator or by accumulation of charge on an electrically isolated body through friction means such as an aircraft or a car by friction with the passing air. Charges can also be accumulated from direct contact with high voltage sources or by transmission from a surrounding ionized atmosphere.
Except in a vacuum, static charges tend to dissipate, or leak off through the conductivity of the surrounding atmosphere. The more conductive the atmosphere, the faster the charge will leak off. In humid weather, the moisture in the air makes the air more conductive than in dry weather when there is little moisture in the air. Thus, we seldom encounter static charge on a humid day and frequently encounter it on a dry day.
Static charges are transferable. Static charge acquired by our clothing is transferred to our body or parts of our body. And, when we approach on object of different potential, we experience an electric discharge as electrons arc from our finger to that object. Static charge can also be transferred from our bodies to tools or other items we contact. These items can in turn impart the charge to a sensitive component, causing damage.
As a general discussion in the elimination of static charge, let us consider the static charge on a plastic film surface. If the surface is placed in an environment of ionized air, it will acquire an equilibrium with the charge of the air. If the surface is of opposite charge, and the air has sufficient energy, the surface will first be neutralized and then equalize with the charge of the air. If there is not sufficient energy in the air, some of the charge will remain on the surface. If the surface is of the same charge as the air, the end charge on the surface will be equal in potential to the end charge on the air.
However, these examples do not occur in bounded isolated reqions, therefore they are continually subject to reactions with the surrounding environment. The initial charge on the air is continually being dissipated to the surrounding environment. The walls or other boundaries of the environment continually absorb and deplete the original charge. Thus, an ionized charge on air rapidly decreases. The film with a surface charge inserted into a chamber with the air at an initial charge, will first equalize with the charge of the air and then lose its charge as the charge on the air dissipates to the chamber walls.
This physical occurance, which can be referenced as the "bleed-off technique", provides an excellent means for eliminating static charge for many applications. However, there are applications where this method is impractical to apply or is not possible to incorporate because of the inclusion of other objectives. One such application, and one which is imposed by the objectives of this invention, is the removal of dust and lint from surfaces of film by use of high velocity air flow simultaneous with the elimination of the static charge, to instantly clean and neutralize a surface passing by the device.
High velocity air passing over a surface is turbulent. Hence, it disrupts singly ionized (positive or negative) neutralizing air making the "bleed-off technique" ineffective. The results are non-uniform leaving portions of the film with a significant residual charge either that of the original charge on the film or that of an induced charge transferred to the film by the ionized air at its initially high potential ionized state. The foregoing occurs whether or not the high velocity air has been ionized either positive or negative with the exception of the application of one specific design technique.
The design exception is the use of alternating fields of positive and negative ionized air. Adjacent fields of opposity polarity quickly neutralize each other, if the opposing fields are properly spaced so that they interact with the surface as well as each other before neutralization occurs. In this way the surface is neutralized along with the annihilation of the charges on the air.
Previous art in this technique is used by Cumming et al, U.S. Pat. No. 4,194,232. In this application air is impressed on a film surface through small holes which have needles projecting through their centers. The needles are supplied with high voltage from an alternating source and serve to ionize the air passing out of the holes. The frequency of the high voltage source is sixty hertz alternating between positive and negative and shows that the field spacing produced by sixty hertz is very effective for this application. Cumming et al also use this technique in another design U.S. Pat. No. 4,213,167.
Attempts have been made to produce the effects of an alternating high voltage air ionizing techniques by using separate constant positive and negative high voltage sources. These techniques incorporate alternately spaced positive and negative ionizing elements. Even though patents on such designs have been issued, U.S. Pat. Nos. 4,498,116 and 4,502,091, to Sournman and 4,333,123, to Moulden experiments show that these designs do not prove to be effective when directed perpendicular to the plane surface, from which the static charge and dust are to be removed.
The deficiency with these systems is that there is not adequate or uniform mixing of the positive and negative ionized regions to bring about a zero net charge in contact with the surface of the film. This leaves the film with residual or induced charges after it has been passed by the system.
However, my previous invention U.S. Pat. No. 4,653,161 does provide the uniform mixing of positive and negative ionized air streams, required, in a vortex chamber so that it does produce a zero net charge at the film surface and does provide neutralization of the stataic charge.
In all of the afore-mentioned systems, the source of air to convect away the dust and lint is provided from a separate and apart compressed air or gas source such as that derived from an air compressor or a high pressure gas bottle. There has been no legitimate attempt to remove the dust and lint through convection by any other convective means, or means that is integral with the performing unit. Although Moulden U.S. Pat. No. 4,333,123 does use a fan in one version of his system, the velocity of the air provided by a fan, as conceived and described, is inadequate to remove the dust and lint from a plane surface. Further, in close proximity his system will not provide complete neutralization of the surface charge, as this system was devised as a static eliminator for a work area (probably for electronic assembly).
Experimentation has shown that air velocities on the order of 100 feet per second are the minimum required to provide the degree of convection that is needed to satisfactorally remove dust and lint from film for the photographic industry. Convective velocities from compressed air or gas sources can be two to three times these velocities or 200 to 300 feet per second. My invention embodies such a self-contained system and is described as follows.